Grid test fixtures, also commonly known as "grid translator fixtures," are used in conjunction with automated, computer-based testing equipment to test the functioning of printed circuit boards. In the process of testing the circuit boards, the test fixture serves as a framework structure that facilitates the establishment of an electrical contact between test points on the circuit board being tested on the one hand, and the testing equipment on the other hand. The number of circuits to be tested on any given circuit board can be quite large, numbering in the tens of thousands, and there typically is a switch for each individual test point. During the testing process the test equipment transfers test signals to selected circuits on the circuit board that is being tested, and a pass, no pass result is obtained. In this way the proper functioning of a circuit board can be quickly checked and verified.
Test fixtures of this type typically include a series of parallel, spaced apart plates, each having a plurality of test probe holes drilled therethrough in a predetermined specific pattern that corresponds to the pattern of test points on the circuit board to be tested. The plates are typically manufactured of a plastic material such as Lexan, G10 or FR4. Test probes, also called "test pins" or "translator pins" extend through the test probe holes in the test fixture plates. The test probes are used to establish electrical contact between test points on the circuit board on one side of the test fixture, and switches interconnecting the probes to the test analyzer on the opposite side of the test fixture. Because the array pattern of the test points on the circuit board is different from the array pattern of the test analyzer, many of the test probe holes drilled through any one plate in the test fixture will be in a slightly different position from the corresponding test probe holes drilled through the next adjacent plate. This results in the test probes being arranged in an image pattern on the circuit board side of the test fixture, and a gridded pattern on the opposite side of the test fixture. Given the many tens of thousands of test probes that may be included in a test fixture, the positioning of the test probe holes drilled through the plates must be precisely controlled. This is typically done with sophisticated computer controlled drilling equipment. But it is also critical that the test fixture is assembled in a controlled and precise manner to minimize any errors in the alignment of the plates. Thus, the plates must be properly oriented with respect to one another so that the test probe holes from one plate to the next are precisely aligned so that the test probes correctly fit through the holes.
In one traditional method of assembling grid test fixtures, the test fixture is assembled with a series of posts spaced around the periphery of the fixture that secure and separate the plates. The posts are constructed of a series of plastic or metal spacers that are inserted between the plates. The spacers separate the plates and hold them in a parallel array. Hollow rods are inserted through bores through the spacers to hold the spacers in the proper orientation. Test fixtures are assembled so that they can be inverted. This allows both sides of the circuit board to be tested. As such, with such traditional assembly arrangements, long connectors such as threaded bolts are typically inserted through the rods in order to hold the entire test fixture together in a fixed position. This manner of assembling test fixtures has several limitations. First, several different sizes of spacers are required because although the plates are parallel, they generally are not all evenly spaced from one another. This results in an increased number of parts that must be kept in inventory. Further, variances in the thickness of the spacers and the plates as a result of manufacturing tolerances for those parts can lead to misalignment of the plates when the test fixture is assembled. Since in any test fixture there are multiple plates, the cumulative effect of size variances in the spacers and the plates can lead to the plates being assembled in a non-parallel orientation. This in turn can lead to misalignment of the test probe holes between plates. Finally, assembling a test fixture in this way can be a time-consuming and tedious job.
An improved method of assembling test fixtures is disclosed in U.S. Pat. No. 5,729,146, entitled Quick Stacking Translator Fixture. In this patent, the spacer and rod type of assembly method is replaced with a series of one-piece stacking towers that have a series of translator plate support surfaces arranged in a stair-step fashion and separated by alignment posts. Each such support surface and corresponding adjacent alignment post is sized to support a translator plate that has been predrilled with a hole sized appropriately for its location on the stacking tower. This manner of assembling the test fixture automatically positions the translator plates at the proper, predetermined level in the text fixture. Outwardly projecting shoulders formed on the alignment posts cooperate with key slots formed on the translator plates to engage the translator plates. When the test fixture is assembled, the stacking posts are rotated to an angular position relative to the plates, causing the plates to pass under the shoulders on the stacking posts so that the shoulders all cooperate to hold the translator plates in position to provide a vertical restraint at each translator plate level.
Despite the improvements made to test fixtures over the years there remains a need for test fixtures that are assembled with precise parallel alignment between the plates. Further, there remains a distinct need for test fixtures that while meeting the requirements of precision in the assembly process are easily assembled in a minimal amount of time.
The present invention approaches the problems associated with assembling test fixtures and fixing the translator plates in position differently from previously known approaches. The test fixture of the present invention utilizes a removable loading plate having a series of loading towers spaced around the periphery of the plate. Predrilled test fixture plates are sequentially loaded onto the loading plate such that the loading towers extend through the holes in the plates. The loading towers have support steps upon which the plates rest at predetermined levels. Each test fixture plate has a series of preformed notches formed around the periphery of the plate. When all of the plates are loaded onto the loading towers, the notches in the plates align and separator posts having cooperative slots are inserted into the notches on the plates. The separator posts engage the notches to fix the plates into position. The top and bottom plates of the test fixture are secured in place with screws extending through the plates and into threaded openings in the separator posts, fixing the test fixture in precise alignment. At this point the loading plate and the loading towers are readily removed from the assembled test fixture.
The test fixture of the present invention is easily assembled. For instance, the separator posts, which lock the plates in a fixed position relative to one another, simply slide into the notches in the outer periphery of the plates. Each separator post has the same configuration, so there is a need to inventory only one part for each specific test fixture arrangement. In addition, the inherent size variances associated with manufacturing tolerances from multiple spacers are eliminated by using identical, unitary support posts. This leads to a high degree of precision in assembling the test fixture.